1. Field of the Invention
The present invention relates to a method and apparatus for quenching machine parts made of metallic materials apt to be cracked or deformed in quenching.
2. Description of the Prior Art
In a typical conventional fog quenching apparatus equipped with "two-fluid" spray nozzles for spraying a mixture of a liquid and a gas, the liquid (water) and the gas (air) are introduced into a mixing chamber for atomization, and the atomized mixture is jetted out from an outer-tip outlet to form a water-air mixture jet (referred to as "spray jet" hereinafter). The flow rate of the water is regulated by means of a needle valve, while the flow rate of the air is regulated by replacing a valve or an inner tip with another. The diameter of waterdroplets in the spray jet is extremely small: on the order of from 5 to 20 .mu.m, showing an excellent atomization. Since the cooling power of this spray jet is relatively small, it is impossible to attain a strong cooling effect, particularly an effect close to that obtained by water cooling. The nozzles of this type have not only a necessity to make the air pressure smaller than the water pressure but also difficulty in regulating the characteristics of the spray jet, together with a complicated structure. In addition, since the water is jetted out from a small gap through the needle valve, the nozzle easily clogs, needing troublesome maintenance and frequent inspection.
In another conventional fog quenching apparatus equipped with spray nozzles similar to the above-mentioned "two-fluid" spray nozzles, the spray nozzles are supplied with water and air having fixed flow rates respectively, and the cooling capacity thereof is regulated by varying the distance between the spray nozzles and the surface of an object to be quenched. However, since a great many spray nozzles are provided, it undesirably takes much time and labor to reset the distance between all the spray nozzles and the surface to be cooled for coping with the variations in shape of objects to be quenched.
Moreover, in the conventional quenching methods employing "two-fluid" spray nozzles, quenching is carried out by means of mixture spray jets with a completely constant mixing ratio, without changing the flow rates of the liquid and the gas, i.e., the mixing ratio therebetween, during the period from the start to the end of cooling, which may cause quenching cracks or deformation.